docs(langfuse): detect local credential helpers

[anpr]

Summary

• add credential-discovery guidance before declaring Langfuse credentials unavailable
• clarify that shell helper functions may not appear in unrelated bash commands
• document invoking discovered helpers through their defining shell, such as zsh -lc
• bump Claude/Cursor plugin manifests to 1.1.2

Verification

• git diff --check origin/main..HEAD
• python3 -m json.tool .claude-plugin/plugin.json
• python3 -m json.tool .cursor-plugin/plugin.json
• verified plugin versions match at 1.1.2
• zsh -lc 'whence -w lf_release_planning_agents lf_alpha'
• zsh -lc 'lf_release_planning_agents traces list --help >/tmp/langfuse-traces-help.txt && head -5 /tmp/langfuse-traces-help.txt'

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